HaKatan: The Issur of talking is not due to the possibility of attraction growing. If that were true, dating would be Assur. The Issur is because, Al Pi Hapsak Shel R’ Moshe, even friendly conversation between members of the opposite gender is inherently Derech Chibah (because since you could be having the same conversation with a guy, it follows that the reason you are having said conversation with a girl is because she is a girl). Read the T’shuvah (EH 4:60). It is clear. Therefore, it stands to reason that cousins, where it is not inherently Derech Chibah, is not an issue. I am saying this myself but it is more than obvious to anyone who reads the T’shuvah.
By your argument, it should be Assur M’ikar HaDin (or at least M’guneh) to speak to your sister.